The British Airways flight at the centre of recent security concerns has been delayed once more by extra passenger checks.
BA supplied security information to the US
BA flight 223 had been due to leave Heathrow for Washington DC at 1505 GMT on Sunday but departed more than three hours later.
The delay was the result of information about the flight demanded by US authorities for what a BA spokeswoman termed "security purposes".
The same numbered flight was cancelled amid security concerns on Thursday and Friday and delayed on Saturday by extra checks before departure.
With BA's weekend flights to and from Riyadh also cancelled over security concerns, Transport Secretary Alistair Darling said air passengers should expect such delays for years to come.
At the same time Prime Minister Tony Blair warned that "the terrorist threat is real right around the world at the present time".
"Nobody is immune from it, you have to remain vigilant."
But he went on: "People should have faith with the authorities, the airlines
and all of us. We are doing the very best with the situation we have.
"In general terms, the terrorist threat is there, there's no point denying that. It was
there before September 11."
The prime minister would not go into detail about the specific concerns that had caused the recent delays and cancellations as he spoke to reporters on the way back from a lightning trip to Iraq.
But he said decisions were taken "on the basis of assessments that are made and you just have to judge each set of circumstances as it comes".
Mr Darling - speaking on BBC One's Breakfast with Frost programme - would also not go into details but stressed the decisions were "justified" on the basis of intelligence warnings.